1. Technical Field
The present invention relates to a sintering method and a thermal insulating member for a porous glass base material suitable for sintering the porous glass base material, which is the raw material for an optical fiber, to achieve dehydration and transparent vitrification.
2. Related Art
Optical fiber uses a porous glass base material, which is primarily quartz glass, as raw material. The porous glass base material is sintered, and resulting optical fiber base material that has undergone dehydration and transparent vitrification is stretched to have a prescribed diameter, thereby obtaining an optical fiber preform. The optical fiber preform is drawn by a drawing machine to obtain optical fiber.
Conventionally, in order to manufacture the porous glass base material, various methods have been proposed. Among these methods, there are external techniques (OVD) that include moving the starting member and a plurality of burners back and forth relative to each other while rotating the starting member, which is obtained by fusing a cylindrical core rod to dummy rods at both ends, around its axis. The resulting structure is then dehydrated and sintered in an electric furnace. This OVD method can be used to obtain a structure with almost any refractive index distribution and to mass produce optical fiber base material with a large diameter, and is therefore used often.
The porous glass base material obtained in this manner has a trunk portion at the center thereof and cone-shaped portions at the ends. A sintering apparatus that includes a heating furnace around a furnace tube is used for the sintering, the porous glass base material is rotated in the axial direction, and the center of the furnace tube moves vertically downward. While this is happening, the heat source of the heating furnace provides heat to achieve the dehydration and transparent vitrification.
The porous glass base material sintering apparatus is shown in FIG. 1, and contains a porous glass base material 3, which is formed by depositing glass fine particles on the outside of the starting member formed by connecting dummy rods 2 to both ends of a core rod 1, arranged within a furnace tube 4. The porous glass base material 3 is heated and sintered by a heater 6 that is protected by an insulation material 5 and arranged on the outer circumference of the furnace tube 4. A driving source 7 causing the porous glass base material 3 to pass through the heating region and a rate control apparatus 8 connected to the driving source 7 for increasing and decreasing the lowering speed are provided on the top portion of the sintering apparatus. A gas inlet tube 9 is attached below the furnace tube 4, and is connected to a source (not shown) that supplies chlorine gas, which is the gas for the dehydration reaction, and a source (not shown) supplying helium gas, which is an inert gas. A exhaust tube 10 is attached above the furnace tube 4.
The sintering of the porous glass base material is realized in the following manner. Chlorine gas and helium gas are supplied from the respective sources (not shown), mixed together while passing through a gas inlet tube 9, and injected into the furnace tube 4. The porous glass base material 3 is controlled by the rate control apparatus 8 to be lowered at low speed while being axially rotated by the driving source 7 and passes through the heating region that generates heat using the heater 6, thereby being sintered to undergo the dehydration and transparent vitrification processes.
At this time, in a conventional sintering apparatus, a large amount of radiant heat scatters up and down from the cone-shaped portions at the ends of the porous glass base material. As a result, the sintering is insufficient near the ends of the base material, and non-uniformity occurs in the processing. Furthermore, natural counter flow occurs in the furnace tube and the pressure fluctuation cannot be controlled, which results in water entering into the furnace tube, leading to a drop in quality.
As a method for solving this problem, Patent Document 1 proposes preventing the scattering of the radiant heat by arranging a ring-shaped thermal insulation board at the cone-shaped portion on the top portion of the porous glass base material. This method does solve the problem described above, but the heat during the sintering causes the thermal insulating board to deform, and this results in a new problem, which is that sufficient performance cannot be achieved unless the thermal insulating board is replaced frequently.
Patent Document 1: Japanese Patent No. 3017990
In order to solve the above problems, it is an objective of the present invention to provide a sintering method and a thermal insulating member for a porous glass base material that can prevent deformation of a thermal insulating board due to heat and also safely prevent scattering of radiant heat when sintering the porous glass base material.